Objective: Listeria monocytogenes (LM) is a gram-positive facultative intracellular pathogen that can cause animal and human listeriosis. In order to use killed LM in vaccination, we compared, in a mouse model, the immunogenicity of LM that were lethally inactivated by gamma-irradiation, traditional heat or formalin treatment.
Methods: BALB/c mice were inoculated intraperitoneally with these killed vaccine candidates. We detected the serum antibody titers with indirect enzyme-linked immunosorbent assay (ELISA) and evaluated the protective efficacy of each vaccine candidates by the resistance to lethal dose challenge of homologous live LM and the effect of bacterial elimination in the spleen and liver of immunized mice. Adoptive transfer of Flow Cytometry sorted T splenocytes from immunized mice to naïve recipients, subsequently challenged with high dose of LM, we determined the possible role of T cell.
Results: The serum antibody level of mice inoculated with gamma-irradiated LM was the highest, up to 1:1280 as determined by ELISA, while the level of mice immunized with heat-killed or formalin-killed LM was 1:640 or 1:160, respectively. The protective efficacy of gamma-irradiated,heat-killed or formalin-killed vaccines candidates were 100%, 35% or 30%. As judged by the bacterial elimination in the organs, mice inoculated with gamma-irradiated LM were the most efficiently protected group. Adoptive T cell transfer assay showed that gamma-irradiated LM can trigger T cell protective immune response.
Conclusion: All the results indicated the superiority of gamma-irradiation over traditional heat or formalin treatment in generating LM killed vaccine candidate. gamma-irradiation may be applied to numerous bacterial vaccine candidates, and could have important potential in development of killed vaccines.